Glow discharge source for elementary analysis

ABSTRACT

The invention relates to a glow discharge source for elemental analysis on solid material samples, which can be operated with a direct voltage, a pulsed direct voltage, or a HF voltage. The aim of the invention is to produce a glow discharge source for elemental analysis on solid material samples, whereby a glow discharge is generated between the material sample ( 4 ) and an anode ( 1 ), by means of a connected electrical voltage source ( 6 ), such that the blind current is minimised and the current flowing through the cooling water is not included in the measurement. Said aim is achieved, whereby a current transformer element ( 9 ), for recording the current flowing between the glow discharge ( 7 ) and the voltage source ( 6 ) is arranged in or on the anode ( 1 ), or the components electrically connected to the anode. The glow discharge source may be used for optical glow discharge spectroscopy (GD-OES), glow discharge mass spectroscopy (GD-MS), or secondary neutral mass spectroscopy (SNMS).

FIELD OF THE INVENTION

[0001] The invention relates to a glow discharge source (GD) for the elemental analysis of solid samples by means of optical glow discharge spectroscopy (GD-OES) or glow discharge mass spectroscopy (GD-MS) or secondary neutral particle mass spectroscopy (SNMS). The inventive glow discharge source may be operated with direct current or with pulsed direct current or with HF voltage.

BACKGROUND INFORMATION AND PRIOR ART

[0002] In the case of the known glow discharge sources, a glow discharge is produced by means of a connected electrical voltage source on the sample of material between the latter and an anode and this is evaluated spectrometrically (EP 0 636 877; DE 41 00 980; V. Hoffmann; H.-J. Uhlemann; F. Präβler; K. Wetzig; Fresenius J. Anal. Chem. (1996) 355: 826-830).

[0003] For the glow discharge sources, operated with direct current (DC-GD), the current usually is measured in the voltage source. When glow discharge sources (RF-GD), operated with a high frequency voltage, the power, running to the source or to the adapting network and reflected by the source, and the high-frequency current and the high-frequency voltage are measured.

[0004] The known current measurement has the disadvantage that a large idle current T_(bl)=T_(bl) ^(an)+T_(bl) ^(ka) is superimposed on the plasma current Tpl. Since water-cooling furthermore is integrated in the known glow discharge sources, a portion of the current T_(wi) ^(wa) flows to ground because of the finite conductivity of the water. As a result, the current, leaking away over the water cooling, is also disadvantageously detected by the measuring equipment so that the plasma current T_(pl), which alone is relevant for the spectroscopy, is distorted.

OBJECT OF THE INVENTION

[0005] It is therefore an object of the invention to configure a glow discharge source for the elemental analysis of solid samples, for which a glow discharge is produced between the sample of material and an anode by means of a connected electrical voltage source, in such a manner, that the idle current T_(bl) is minimized and the current T_(wi) ^(wa), flowing over the cooling water, is not measured.

[0006] Pursuant to the invention, this objective is accomplished with the glow discharge source, which is described in the claims.

[0007] Pursuant to the invention, a current transformer component for detecting the current flowing between the glow discharge and the current source, is disposed at or in the anode or the components connected electrically with the anode.

[0008] Pursuant to appropriate embodiments of the invention, the current transformer component may be a coil or a Hall generator. The current transformer component may also be an ohmic resistance, which is inserted in the connecting piece of the anode and connected with an ammeter.

[0009] In the event that an HF voltage source is used, the current transformer component advantageous is surrounded by HF shielding.

[0010] Compared to known sources, the inventive glow discharge source is distinguished by the fact that the current measurement is integrated in the source, since the current, flowing in the region of the anode, which is grounded at the generator, is converted into a measurement signal. By these means, only the current, supplied to the glow discharge or the plasma, is measured and conditions, reproducible for different samples of material, are ensured. With that, the quality of the spectrometric results is improved significantly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention is explained in greater detail below by means of examples and associated drawings, in which

[0012]FIG. 1: shows the functional diagram of a conventional glow discharge source, operated with HF, in a sectional representation,

[0013]FIG. 2: shows an inventive glow discharge source, operated with HF, in sectional representation with an integrated induction coil and

[0014]FIG. 3: shows an inventive glow discharge source, operated with direct current, in sectional representation with an integrated ohmic resistance.

[0015] The conventional glow discharge source for the GD-OES, shown in FIG. 1, is constructed one anode 1 and two cathode plates 2; 3, a sample of material 4 being clamped between the cathode plates 2; 3. The cathode plates 2; 3 are equipped with cooling channels, through which water flows as coolant. The anode 1 has an anode-connecting piece 5, which discharges over the sample of material 4, forming a space.

[0016] An HF voltage source 6 is connected to the anode 1 and the cathode plates 2; 3. By these means, a glow discharge 6, with which the surface of the sample of material 4 is removed by sputtering, is maintained between the material sample 4 and the end of the anode-connecting piece 5. The glow discharge 6, into which the chemical elements, sputtered from the material sample 4, are brought, is then analyzed by means of OES.

[0017] For this glow discharge source, the current is measured in a known manner in the connecting lead 8 by means of a current transformer. This measurement is associated with the already indicated distortion of the measured value by the idle current T_(bl), which is also measured and by the current T_(wi) ^(wa) leaking away over the cooling water.

WAYS FOR CARRYING OUT THE INVENTION EXAMPLE 1

[0018] The first example of an inventive glow discharge source, shown in FIG. 2, differs from the conventional source owing to the fact that an induction coil 7 is disposed around the anode-connecting piece 5. The induction coil 9 is surrounded by HF shield 10. As endeavored, only the HF current, flowing from the glow discharge at the surface of the anode connecting piece 5 to the voltage source 6, is detected inductively with the induction coil 9.

EXAMPLE 2

[0019] In this example, which relates to an inventive glow discharge source supplied by a direct current source 11, an ohmic resistance 12 is inserted in the anode-connecting piece 5. This resistance 12 is connected with an ammeter. With this glow discharge source also, only the current, flowing from the glow discharge to the voltage source, is detected. 

1. A glow discharge source for the elemental analysis of solid samples of material by means of optical glow discharge spectroscopy (GD-OES) or glow discharge mass spectroscopy (GD-MS) or secondary neutral particle mass spectroscopy (SNMS), a glow discharge being produced by means of a connected electrical voltage source on the sample of material between the latter and an anode and evaluated spectrometrically, wherein a current transformer component for detecting the current flowing between the glow discharge (7) and the voltage source (6; 11) is disposed at or in the anode (1) or connected electrically with the anode.
 2. The glow discharge source of claim 1, wherein the current transformer component is a coil (9).
 3. The glow discharge source of claim 1, wherein the current transformer component is a Hall generator.
 4. The glow discharge source of claim 1, wherein the current transformer component is an ohmic resistance (12), which is inserted in the connecting piece (5) of the anode (1) and connected with an ammeter.
 5. The glow discharge source of claim 1, wherein, in the event that an HF voltage source (6) is used, the current transformer component is surrounded by HF shielding (10). 